I wonder how many of the current userbase will login and transfer the bookmarks, I guess theres a lot of value in the historical accounts that won't transfer with the deal..
This may be true, yet its a great indicator for both parties to see how many users are willing/current in the system. It certainly can be a line in the sand fir future comparisons.
i wonder how much of the current userbase hasn't already switched to pinboard.
Since it's easy to calculate Pinboard's user numbers, apparenty most of them. Browser bookmark sync is probably a much bigger alternative.